Transformation of fibroblasts from many species is accompanied by increased release of serine protease activity that activates plasminogen (plasminogen activator, "PA"). Amounts 30 to 100 times those produced by normal fibroblasts are seen following transformation with chemical or viral agents and in primary cultures of spontaneous neoplasms of animal and human origin. The enzyme is not induced by non-oncogenic viruses, and cells infected with a mutant of Rous sarcoma virus temperature-sensitive for transformation release increased amounts of PA only at the permissive temperature. Studies using exogenous proteases and protease inhibitors have produced evidence that PA or related activities may be causally associated with expression of the transformed phenotype. These and other observations suggest that proteases may be involved in regulating cell differentiation by allowing cells to change their surfaces or environment or those of their neighbors. A number of proteases have been shown to be mitogenic for various types of cells, and neoplastic cells that secrete large amounts of such enzymes are thus potentially self-stimulating systems. This mechanism may account, at least in part, for the unregulated growth of transformed cells. Increased levels of plasminogen activators have also been associated with tissue development and remodeling and with inflammatory processes. It would therefore seem important to investigate these enzymes in detail, especially with respect to their growth-promoting properties and the way in which their synthesis is controlled. The specific goals of this project are to continue efforts to obtain adequate amounts of highly purified plasminogen activator from Rous sarcoma virus-infected chick embryo fibroblasts (RSV(CEF)); to determine whether plasminogen activator has a role in cell transformation; to determine the manner in which plasminogen activator synthesis and secretion are controlled in normal and transformed cells; and to prepare antibodies and nucleic acid probes to be used as useful reagents for the pursuit of these studies.